Designing a Gas
Detection System
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Why Gas Detection?
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Safeguard Life and Property.
– Provide Early Warning of Hazardous Conditions.
– Provide Opportunity for Evacuation and Notification from Re-entry
– Provide Time for Intervention and Correction.
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Trigger Facility Protection Systems.
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Ventilation, Water Mist, Fire Suppression.
Satisfy Local Fire Code and Provide Insurability.
Address Real and Perceived Safety Concerns.
Note: Gas Detection is “Recommended Practice,” “Required by Code,”
or “Required by Law.”
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Points to Consider
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Understand the application
Identify potential danger points
Establish design goals
Determine gas characteristics
Profile the plant and potential release scenarios
Other elements in selecting gas detection systems
Actual placement of detection
– Indoors
– Outdoors
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Understand The Application
• The gases to be monitored
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Toxic (STEL, TLV, TWA)
Combustible (LEL, UEL)
Exposure limits
Density and Other Gas Properties
• Local and federal regulations
– Uniform Fire Code
– Code of Federal Regulations
– Local Fire Marshal
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Identify Potential Danger Points
– Release Points – sensors should be placed as close as possible to potential
leak sources.
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Seals and flanges, fittings and welds
Expansion joints and gaskets
Engine combustion
Storage, loading and unloading areas
Runoff areas
Decomposing materials
– Receptor Points – a gas detection notification system should protect any
person, property or equipment that may come in contact with harmful gases.
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Wind direction
Ventilation systems
Run off areas
Confined spaces
Communities and facilities
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Establish Design Goals
Initiate a response based on an early warning of a potential problem.
 Notification or annunciation – method of warning
 Ventilation control
 Process shutdown
 Evacuation and emergency response
 Amount of confinement - over pressurization and accumulation
 Run-up distance – speed of flame increases with distance
 Amount of congestion or obstacles
 Fuel quantity and mixing
 Margin of safety – distance between leak source and receptors
 Plant safety process
 Insurance requirements
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Determine Gas Characteristics
• LEL, UEL, Toxicity
• Vapor density
– Density differences with temperature
• Cryogenic liquids, flammable liquids
– Low density gases displacing ambient density gases (helium vs. oxygen)
– Gases under pressure will condense in areas where vented first
• Gases changing composition – dry ice
• Toxicity vs. flammability (MTBE 40 PPM 1.6% LEL)
• Hydrolyzed (BF3, F2)
• Pyrolyzed (NF3)
• Flash point - the lowest temperature at which a liquid can form an ignitable
mixture in air near the surface of the liquid. The lower the flash point, the easier it
is to ignite the material.
• Rate of evaporation and dispersion characteristics
• Gas mixing (fuel, oxygen, ignition source)
• Oxygen enriched environments
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Gas Hazards
There are three main types of gas hazard
1.
Flammable
– Risk of fire and or explosion,
e.g. Methane, Butane, Propane
2.
Toxic
– Risk of poisoning,
e.g. Carbon Monoxide, Hydrogen Sulfide,
Chlorine
3.
Asphyxiant
– Risk of suffocation,
e.g. Oxygen deficiency, Nitrogen, Carbon
Dioxide
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Flammable Risk
•
Fire Triangle
Three factors are always needed to cause
combustion:
FIRE
1. A source of ignition
2. Oxygen
3. Fuel in the form of a gas
or vapour
fuel
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Flammable Risk
• The operation of a car’s choke
illustrates an important part of
flammable gas hazards
• All flammable gases are only
ignitable over their flammable range
• Flammable gases tend to be
measured in percentage of their
explosive Limit (%LEL)
100% v/v gas
0% v/v air
too rich
flammable
range
U.E.L. (upper
explosive limit)
L.E.L. (lower
explosive limit)
too lean
0% v/v gas
100% v/v air
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Toxic Risk
• Some gases are poisonous and
can be dangerous to life at very
low concentrations.
• Some toxic gases have strong
smells like the distinctive ‘rotten
eggs’ smell of H2S
• Others are completely odourless
like Carbon Monoxide
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Toxic Risk
• The measurement most often used
for the concentration of toxic gases
is parts per million (ppm).
• For example 1ppm would be
equivalent to a room filled with a
total of 1 million balls and 1 of those
balls being red. The red ball would
represent 1ppm.
1 million balls
1 red ball
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Profile the Plant and Potential Release Scenarios
Gas sensors should be placed to ensure that a quantity of gas will past by them
in all normal release scenarios.
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Identify physical features of plant
Identify ventilation tracks
Identify escape routes
Protect entrances to areas
Mark escape routes
Identify wind directions
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Other Elements
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Accessibility for calibration and maintenance
Wiring and installation
Environmental conditions
EMI and RFI
Alarm levels
Exposure limits
Oxygen levels – some toxic gas electrochemical sensors require a
minimum oxygen level to function. All catalytic bead combustible
detectors require oxygen to work.
• Be aware of poisoning and inhibiting factors
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Interior Detector Placement Guidelines
• Operate Detectors Within their Temperature Limits. Use Sample Draw or Duct
Mount Configurations When Needed
• Water, Moisture, Dust and Dirt May Affect Performance. Minimize Exposure and
Protect From Adverse Conditions
• Locate Detectors With Respect to Grade, Floor, or Operating Level - According to
Building Design, HVAC System, Characteristics of Potential Leak
• When Monitoring Specific Equipment, Place Detectors Near (12 Inches) Pump,
Seal, Tank, Valve, etc.
• Sensitivity of Detector Depends on Proximity to Leak. Adjust Alarms if Earlier
Annunciation is Required
• Mount Detectors Securely, Independent of Vibration, With Weather Shield Facing
Downward
• Conduct Smoke Trace Behavior Studies If in Doubt
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Detector Spacing Indoors
• There are Few Published Guidelines and No Standards Indicating Area or
Volume Effectively Protected By a Diffusion Sensor. There is a Corollary in
Fire Protection
• UL Suggests a 900 Ft2 Ceiling Space Per Smoke Detector, Which is a 30 Ft.
Square or 15 Ft. Radius
• Using This Base Guideline, the Total Number of Detectors Must be Based on
– Gas Dispersion Characteristics and Air Movement
– Potential Leak Source Locations and Characteristics,
– Sources of Ignition Locations
– Interior Space Division by Walls or Barriers
– Economics of the Procurement
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Detector Spacing Indoors, Cont’d.
• UL Suggests a 900 Ft2 Ceiling Space Per Smoke Detector
15 Feet
15 Feet
30 Feet
Detector
Detector
30 Feet
References
NFPA 72 E, Standard On Automatic Fire Detectors
Schaeffer, M.J., “The Use of Combustible Detectors in Protecting Facilities from
Flammable Hazards,” ISA Transactions, Volume 20, No. 2, Instrument Society of
America 1981
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Indoors
40 feet
Door
Ceiling
Ventilation
20 ft
Natural Gas
S
T
O
R
A
G
E
Lab Bench
Cl2
Door
LN2
Window
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Outdoor Detector Location Guidelines
• Use same considerations outdoors as indoors.
• Consider Angle and Direction of Prevailing Wind
• The Orientation of Structures and Surrounding Terrain with Regard to
Shielding Affects
• The Proximity of Large Quantities of Toxics to Personnel and Equipment,
Which May Require Added Detectors to Isolate the Two
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Outdoor Detector Location Guidelines
Possible Entrapment of Leaking Gases and
Vapors Within Columns, Low Lying Areas or
Confined Spaces
Sources of Ignition and Processes With
Fugitive Leak Potential Are Considered for
Detector Placement
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Outdoor Detector Location, Cont’d.
• Heavier Than Air Gases or Vapors: Vapor Density >1
– The Preferred Location for Detectors is ~18 Inches Above Grade. For
Liquid Spills, As Close to the Vapor/Liquid Interface as Possible, and Still
Allow for Detector Calibration
• Lighter Than Air Gases and Vapors: Vapor Density <1
– The Preferred Location for Detectors is About 6 to 8 Feet Above Grade or
Operating Level, With Special Attention Being Paid to Air Currents,
Structures, Roofed Areas, Etc.
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Detector Spacing Outdoors
• Few Guidelines Exist for Detector Placement
• An Increased Grid Density is Used Outdoor Presumably
Because of the Greater Potential for Leak Dilution
• Thus, More Detectors Are Required, and Potential Leaks Are
Encircled, to Account for Wind Shifts
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Detector Spacing Outdoors

Repeat Outdoor Grid Pattern as Conditions
Warrant. Focus on Potential Leak Sources
for Additional Detectors if Required
Detector
10-15 Feet
Detector
10-15 Feet
20-30 Feet
20-30 Feet
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General Location Considerations
• Toxic Gases and Vapors:
– Identify Potential Leak Sources, Work Areas, and Exit Points. Understand Where
People are Performing Their Work and Place Detectors Between Probable Release
Points and the Work Area
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General Location Considerations
• Allow Access for Sensor Calibration and Replacement.
– Sensors Have a Finite Life - Calibrate and Maintain Regularly!
• Always Locate Detectors Using “Local Conditions Knowledge,”
and Lighter or Heavier Than Air Principles
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General Location Considerations
• Locate Detectors Within Their Temperature Rating
– Avoid Exposure to Sources of High Radiant Heat
• Keep Detectors Away From Moisture and Chemicals
– Avoid Vibration and Mechanical Shock Hazards
• Observe Recommended Wiring and Tagging Practices
• Use Shielded Cable Whenever Possible and Follow Correct Grounding
Practices (NFPA 70)
• Observe Proper Detector Mounting Orientation
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Detector Location and Area Coverage Map
IMPORTANT
Detectors should be located close to any potential leak source
and between leak source and any potential source of ignition
existing at the monitored site.
XNX-Optima
FILL LINE
DETECTOR LOCATED ON
TOP OF TANK NEAR VALVE
OR FLANGE ON FILL LINE
FILL LINE
TANK SUPPORT LEGS
TOP VIEW OF STORAGE TANK
OUT FLOW
SIDE VIEW OF STORAGE TANK
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DETECTORS SHOULD BE LOCATED BELOW TOP OF DYKE WALL
FOR VAPORS THAT ARE HEAVIER THAN AIR.
Sensor should be approx 12-18 in above grade. These vapor densities
are greater than air.
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TOP VIEW OF STORAGE TANK
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Detector Location and Area Coverage Map
IMPORTANT
Detectors should be located close to any potential leak source
and between leak source and any potential source of ignition
existing at the monitored site.
XNX-Optima
FILL LINE
DETECTOR LOCATED ON
TOP OF TANK NEAR VALVE
OR FLANGE ON FILL LINE
FILL LINE
TANK SUPPORT LEGS
OUT FLOW
SIDE VIEW OF STORAGE TANK
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DETECTORS SHOULD BE LOCATED BELOW TOP OF DYKE WALL
FOR VAPORS THAT ARE HEAVIER THAN AIR.
Sensor should be approx 12-18 in above grade. These vapor densities
are greater than air.
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Detector Location and Area Coverage Map
IMPORTANT: This is intended as a General Application Note and NOT as the sole source of
information in determining quantity and location for detector placement. Consult additional
resources when developing a monitoring system. Additional information is available for developing
Combustible Gas Detection systems, such as: The National Fire Protection Association, NFPA 52
and the Instrument Society of America, ISA-RP12.13-Part II-1987. Services are also available from
Professional Safety Engineering Firms and should be utilized whenever necessary
TOP VIEW OF THREE AND TWO TANK AREA
SIDE VIEW OF TANK
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4
5
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DETECTORS SHOULD BE
LOCATED BELOW TOP OF
DYKE WALL FOR VAPORS
THAT ARE HEAVIER THAN AIR.
Sensor should be approx 12-18 in.
above grade..
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IMPORTANT
Detectors should be located close to
any potential leak source and between
leak source and any potential source of
ignition existing at the monitored site.
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Detector Location and Area Coverage Map
TOP VIEW OF UNLOADING FACILITY
R.R. SPUR NO. 13
R.R. SPUR NO. 12
IMPORTANT
Detectors should be located close to any potential leak source and between
leak source and any potential source of ignition existing at the monitored site.
NOTE: IF PIPING
FUNCTIONS OR VALVES ARE
ABOVE THE TANK CAR A
DETECTOR MAY BE
MOUNTED PART WAY DOWN
THE WALL TO DETECT
FALLING VAPORS FROM A
LEAK SOURCE
I
I
I
I
NOTE: DETECTORS SHOULD
BE LOCATED LOW, CLOSE TO
GRADE FOR VAPORS THAT
ARE HEAVIER THAN AIR.
NOTE: Sensor should be approx.
12-18 in. above grade. These vapor
densities are heavier than air.
SIDE VIEW OF UNLOADING FACILITY
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Publications to Reference
• Chemical Weekly, 2008, “Key Considerations when Designing a Gas
Detection System”
• ISA Recommended Practices
• ACGIH: Annual TLV and BEI Guide
– 1330 Kemper Meadow Drive, Cincinnati OH 45240-1634
• NFPA/ANSI Guides, Standards and Practices
– 1 Batterymarch Park, Quincy MA 02269-9101
• NIOSH: Pocket Guide to Chemical Hazards
– Available by FAX Request to (513) 533-8573
• OSHA, CFR 29, Section 1910.1000, Subpart Z“Working in Confined
Spaces.”
– NIOSH1 Publication 80-106
• “A Guide to Safety in Confined Spaces.”
– NIOSH Publication 87-113
• “ALERT: Request for Assistance in Preventing Occupational Fatalities in
Confined Spaces.”
– NIOSH Publication 86-110
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The End
Questions?
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